Radiation tomography apparatuses that image the distribution of a radioactive agent are deployed in medical institutions. The following describes a specific configuration of such a radiation tomography apparatus. A conventional radiation tomography apparatus includes a detector ring including radiation detectors annularly arranged and detecting radiations. This detector ring detects a pair of radiations that are emitted from a radioactive agent within a patient and are opposite in direction to each other (an annihilation radiation pair). Such an apparatus is called a positron emission tomography (PET) apparatus.
FIG. 17 illustrates a typical PET apparatus. The PET apparatus is of the stationary type fixed to a floor of a inspection room, and a patient is introduced from an opening provided in the PET apparatus. An image obtained by this PET apparatus means the distribution of the radioactive agent within the patient. Consequently, the distribution of a highly-concentrated radioactive agent on this image would not always definitely indicate which organ of the patient.
Given these circumstances, as illustrated in FIG. 17, an image imaging apparatus including both the PET apparatus and a CT apparatus has been devised and widely used. The CT apparatus of this imaging apparatus gives a morphological image showing the internal structure of the patient, whereas the PET apparatus gives a functional image showing the distribution of the radioactive agent. When morphological image and the functional image are displayed in a superimposed manner, a viewer of the images can tell which part of the patient the radioactive agent has been distributed.
The following describes the reason why the PET apparatus and the CT apparatus are arranged close to each other. Suppose that the PET apparatus and the CT apparatus are prepared in different inspection rooms, for example. In such a configuration, a morphological image is first taken with the CT apparatus, then the patient is moved to another room, and a functional image is taken with the PET apparatus. In such as a configuration, the morphological image and the functional image cannot be precisely superimposed on one another. This is because the postures of the patient when imaged by the respective apparatuses are different from each other. To reduce such image discrepancy, the PET apparatus and the CT apparatus are provided close to each other. By doing so, once the patient is placed on a patient couchpatient couch, there is no need to take down the patient from the patient couchpatient couch until capture of the morphological image and the functional image ends. The apparatus in FIG. 17, allows the CT and PET apparatuses to finish imaging while the patient maintains the same posture, such that the obtained morphological image and functional image can be precisely superimposed on one another.
However, imaging with the CT apparatus creates a large amount of exposure dose and is not suitable for pregnant women, children, and the like, who have high radiosensitivity. Hence, apparatuses gradually in common in recent years are a PET/MRI apparatus whose CT apparatus illustrated in FIG. 17 is replaced with an MRI apparatus, and a completely integral type PET/MRI apparatus in which detectors of a PET apparatus are contained within an imaging field of view of an MRI apparatus. However, the PET/MRI apparatus is large-scale in configuration, and the price of the apparatus itself tends to be extraordinarily high. Some medical institutions already having an MRI apparatus may desire a PET apparatus that can be placed by the side of the MRI apparatus. In view of such a need, a mobile type PET apparatus as illustrated in FIG. 18 has been devised in recent years. In this mobile type PET apparatus, when a patient is placed on a patient couch included in the MRI apparatus, a functional image can be taken without moving the patient from this patient couch (see NON-PATENT DOCUMENT 1, for example).
The patient couch included in the MRI apparatus has a long, narrow shape to fit the patient. The patient couch is movable in a longitudinal direction (the body axis direction of the patient placed on the patient couch). This is because the patient is required to be introduced into a gantry of the MRI apparatus. The mobile type PET apparatus is brought close to and separated from the patient couch in the lateral direction of the patient couch (the body side direction of the patient placed on the patient couch) as indicated by the arrow in FIG. 18. When imaging is performed with the PET apparatus, the patient and the patient couch are required to be introduced inside the detector ring. In a conventional configuration, for the purpose of introducing the detector ring from the body side direction of the patient, the detector ring is provided with a clearance A. This clearance A makes the detector ring form into a C shape.
FIG. 19 illustrates how the patient is actually introduced inside the detector ring. The detector ring is provided with the clearance A, whereby the patient and the patient couch are introduced into the center of the detector ring without being interfered with the detector ring.
FIG. 20 illustrates the transformation of the detector ring performed before starting PET imaging. In other words, the detector ring immediately after the introduction of the patient illustrated on the left side in FIG. 20 is transformed so that the transformed detector ring vertically sandwiches a patient M therebetween as illustrated on the right side in FIG. 20. The detector ring of the mobile type PET apparatus forms a C shape by combining two arc members with each other. These members can rotate and move about the central point of the detector ring. With this movement, the clearance A appearing on the right side of the detector ring is divided into right and left.
Essentially, the detector ring is desirably annularly shaped. If it is not so shaped, annihilation radiation pairs are unevenly detected, making it difficult for a clear functional image to be taken. However, the clearance A has to be provided to the mobile type PET apparatus because a path is required to introduce the patient through the path. Consequently, in the conventional configuration, after the patient is introduced, the detector ring is transformed to divide the clearance A, whereby the influence of the clearance A appearing in the functional image is ingeniously reduced to a minimum.